Some wireless communications systems utilize multiple-input and multiple-output (MIMO) technology. MIMO technology involves the use of multiple antennas for both the transmitter and a receiver so as to improve communication performance, such as by offering significant increases in data throughput and link range without additional bandwidth or transmit power as a result of its higher spectral efficiency as measured, for example, in bits per second per hertz of bandwidth, and link reliability or diversity. For example, systems that operate in accordance with Releases 8, 9 and 10 of the Long Term Evolution (LTE) specification support MIMO, that is, MIMO transmissions from a transmit point (which can be also represented by a base station, such as a evolved node B(eNB)), to one or more mobile terminals. Indeed, Releases 8 and 9 of the LTE specification support four stream transmission, which could result in 4×4 MIMO in the context of four transmission (Tx) antennas and four reception antennas. In addition, one stream and two stream beamforming from a maximum of 8Tx antennas have been introduced in Release 8 and Release 9, respectively. Release 10 of the LTE specifications extends support for downlink MIMO for up to eight stream transmissions and therefore up to at least 8×8 MIMO in the context of eight transmission antennas and eight reception antennas. Additionally, Release 10 of the LTE specification provides enhanced support for multi-user MIMO as well as support for seamless switching between single and multi-user MIMO.
To facilitate closed-loop MIMO communications, reference signals may be used for channel state information estimation. This is especially valid in frequency division duplexing (FDD) systems where channel reciprocity cannot be exploited, as it is the case in time division duplexing (TDD). For example, in the context of downlink MIMO, channel state information feedback may be provided via channel state information reference signals from a mobile terminal to the transmit point. The channel state information reference signals may be transmitted periodically, may have a low overhead and high reuse factors, especially in instances in which the number of transmission antennas is relatively low. Reference signals are mapped to the transmit point antenna ports. Antenna ports may have a one to one mapping to the physical transmit antennas of the transmit point, or may characterize multiple transmit antennas of one transmit point. Indeed, Release 10 of the LTE specification includes channel state information reference signals (CSI-RS) as well as physical downlink shared channel (PDSCH) resource element (RE) muting, which complement the CSI-RS operation in an instance in which accurate inter-cell channel estimation is desired. All LTE Releases make use also of Common Reference Signals (CRS) which are sent in every subframe and may characterize a maximum of 4 antenna ports.
By way of example, FIG. 1 illustrates resource elements usage for normal cyclic prefix (CP) in frame structure type 1 (which applies to FDD) as specified in Release 10 of the LTE specification for one physical resource block (PRB). As shown in FIG. 1, resource elements, that is, respective elements of the orthogonal frequency division multiplexing (OFDM) time frequency grid represented by the PRB, may be allocated for common reference signals (CRS) ports #1 and #2, CRS ports #3 and #4, demodulation reference signal (DMRS), port #5 if so configured for Release 8 of the LTE specifications, DMRS for Releases 9 and 10 of the LTE specifications, physical downlink control channel (PDCCH) and physical downlink shared channel (PDSCH). The physical resource block also includes resource elements allocated for CSI-RS. In this regard, the CSI-RS resource elements are designated with numbers, e.g., 0, 1, . . . 7, with the alphabetical suffixes, e.g., a, b, . . . t, indicating the different transmit points to which the CSI-RS resource elements are directed. For example, the 0b, 1b resource elements are a pair of CSI-RS resource elements mapped to the transmit antennas of a transmit point. They can be indicated in a dedicated way to one or more mobile terminals
The density of the CSI-RS pattern in accordance with Release 10 of the LTE specification is one CSI-RS resource element per antenna port per PRB. By way of example, FIG. 1 illustrates the PRBs for two antenna ports, four antenna ports and eight antenna ports with two REs, four REs and eight REs allocated for CSI-RS, respectively. Similarly, FIG. 2 illustrates the CSI-RS pattern for extended CP in frame structure type 1 (FDD) specified in Release 10 of the LTE specification for one PRB. As described above in conjunction with FIG. 1, FIG. 2 illustrates the PRBs for two antenna ports, four antenna ports and eight antenna ports. Further, FIG. 3a illustrates the CSI-RS pattern for normal CP in frame structure type 2 (which applies to TDD), while FIG. 3b illustrates the CSI-RS pattern for extended CP in frame structure type 2 (TDD) with these patterns being utilized in addition to the patterns utilized for frame structure type 1, as shown in FIGS. 1 and 2.
PDSCH RE muting is used in a complementary way to the CSI-RS if accurate inter-transmit points channel state information is desired. The same CSI-RS patterns are used for the muted REs.
The density of one RE for CSI-RS per PRB per port may be sufficient for single-cell CSI estimation which may, in turn, allow the base station to control the antenna weights to achieve a precoding gain. However, it may be desired for the mobile terminal to provide feedback information not only regarding CSI estimation, but also regarding interference, such as the interference covariance matrix that may be required for channel quality indication (CQI) and rank estimation. In this regard, the density of one resource element for CSI-RS per PRB per port may be insufficient for interference covariance estimation or, at least, may lead to an undesirably large error in the estimation of the interference covariance which, in turn, may cause erroneous CQI and erroneous rank feedback which may lead to a poor selection of transmission scheme, both modulation and coding scheme (MCS) and rank, by the base station. These poor selections may, in turn, lead to throughput loss at the system level.
In Release 10 of the LTE specification, the mobile terminal may rely on CRS for interference estimation. However, CRS may have a reuse factor of 3 with two or more transmission antennas such that CRS collisions of neighboring cells are unlikely to be avoided. This scenario may cause less accurate CQI and rank estimation in typical fractional load scenarios than is desired. Thus, CQI and rank estimation could benefit by the use of CSI-RS. However, the current CSI-RS density is too low to provide the desired channel state information/interference accuracy.
Furthermore, there is an ongoing effort to support heterogeneous networks which exhibit difficult interference characteristics. Thus, there is a tendency in 3rd Generation Partnership Project (3GPP) networks to reduce CRS usage, as CRS may be problematic from the inter-cell interference perspective. In a backwards compatible manner, this CRS reduction can be accomplished, for example, with Multimedia Broadcast over a Single Frequency Network (MBSFN) sub-frames or by the definition of new extension carriers (without CRS) in relation to carrier aggregation. Additionally, the reference signals provided by release 10 of the LTE specifications, such as the CSI-RS, have a relatively low overhead with respect to CRS, which provides a further incentive to limit the role of CRS going forward. Additionally, in transmission mode 9 of Release 10 of the LTE specification, the number of CRS can be adjusted by turning off some of the ports, thereby potentially leading to the utilization of only one CRS port at a minimum. As such, it cannot be assumed that the mobile terminal will always have sufficient CRS to perform interference estimation in the future.
Coordinated multi-point transmission (CoMP) will require a mobile terminal to measure from the CSI-RS the channel from transmission points that are geographically separated from a reference transmission point, such as a serving cell. These measurements may include propagation delay differences that show up in the effective frequency-domain channel as an additional linear phase rotation. Thus, the mobile terminal may be required to estimate a more frequency-selective channel and/or to estimate the timing differences between the different transmission points. Such estimation and the subsequent feedback to the base station would also require more use of the reference signals, such as the CSI-RS, and a greater density of the reference signals within a PRB.
Notwithstanding the potential for increased utilization of the CSI-RS, efforts to simply increase the density of the reference signals, such as the CSI-RS, within a PRB in order to support the additional feedback that is desired creates backwards compatibility issues with legacy mobile terminals, such as those mobile terminals that operate in accordance with prior releases of a respective specification. In this regard, since these legacy mobile terminals would not be aware of the additional reference signal transmissions, such as the additional CSI-RS, the additional reference signal transmissions may disadvantageously interfere with data transmissions of the legacy mobile terminals.